Toughened epoxy resin composites have been suggested for use in structural applications where materials such as aluminum were previously employed.
A problem in the use of structural composites based on organic polymers, is that some organic polymers, particularly epoxies, are suspect as hazards due to their flammability properties when exposed to fire and/or heat. This hazard is particularly noted when such an organic polymer based composite is used in an aircraft environment. The U.S. Government has promulgated standards which are required of materials that may be used in the aircraft industry. Many of the polymer materials cannot pass the government standard and, unless made more flame retardant, can not be approved for use in aircraft.
A U.S. Government standard for flammability resistance is set out in FAR 25,853 et seq. (14 CFR Ch 1) and are incorporated herein by reference. Three of the standard tests described in Appendix F to FAR 25.583 and 25.855 require a twelve and a sixty second vertical burn and a thirty second 45 degree angle burn. In these tests, the flame, usually a bunsen burner type flame, impinges on the material. To pass the sixty second vertical burn test the material after a sixty second flame application, if ignited must self-extinguish within fifteen seconds, the average vertical burn length must not exceed approximately 15.2 centimeters, and the average melt drip extinguishing time must be less than or equal to three seconds. This means that the material must put itself out, that the flame must not spread throughout the material and that the material cannot be a source of fire spread to other areas.
Epoxy resin materials are known for their bonding properties. Epoxy materials, however, have been limited in applicability due to the hazard presented because of their flammability and/or smoke generating properties.